In developed and developing countries, primary treatment and disinfection of waste water discharges from collection systems and waste water treatment facilities is the first step to improving water quality. As the countries continue to advance, secondary and tertiary waste water treatment processes are added to provide additional treatment of the primary effluent.
Primary treatment removes large solids via screening and gravitational settling to remove light and dense solids, allowing neutrally buoyant matter to pass into the secondary treatment process or receiving body of water. Primary treatment utilizing gravitational settling or clarification is recognized as removing 20-33% of the organic load as measured in Biochemical Oxygen Demand (BOD). Secondary treatment removes another 50+% of the organic load by converting the BOD to biomass (bacteria) and CO2.
Secondary treatment provides an environment of adequate temperature, volume, mixing, and oxygen or the absence of oxygen in anaerobic processes to sustain the bacterial population necessary to consume the BOD and nutrients remaining in the waste water after primary treatment. New organic matter enters the treatment facility continuously so a portion of the existing bacterial population is removed from the process to promote the growth of new bacteria. The effectiveness of primary treatment directly affects secondary process or the receiving body of water if discharged from the collection system.
Primary clarifiers or settling basins are recognized as being the most economical means to reduce BOD as there is little energy required and no biomass to maintain. Primary treatment has no biomass therefore no aeration energy; no process controls to monitor the biomass to determine the health of the biomass by the types and quantity of the bacteria; no need to separate and remove or waste the bacteria by moving to a side-stream digester; no need to aerate the digester; and no need to dewater and dispose of the surplus bacteria, also called secondary sludge. The lack of complexity of primary treatment is well suited for developing nations and begins an effective recovery of their surface waters and aquifers resulting in reduced health issues.
Prior art primary clarifiers may be circular or rectangular tanks and are volumetrically and geometrically sized to provide a horizontal fluid velocity lower than the solids settling velocity. The horizontal travel time and distance of the liquid from the inlet to the effluent weir must be greater than the settling time and distance of the suspended solids so that solids settle to the bottom of the tank prior to reaching the elevated effluent weir. These settled solids contain a majority of the BOD in raw sewage. This is an important first stage because the more solids that exit the primary clarifier (or if there is no primary clarifier), the higher the BOD entering the secondary treatment process or the effluent-receiving body of water. The higher the BOD entering the secondary treatment process, the larger the required secondary process equipment and tanks, the more biomass required, generated, and disposed of, the more processing energy that must be expended. The higher the BOD of the effluent stream entering the receiving body of water the greater the eutrophication of the water body and the more detrimental to the health, due to poor disinfection.
An example based on standard design parameters to achieve 33% BOD reduction is shown as follows:
Minimum depth=10′; Surface Overflow Rate=1,000 Gallons per day (GPD)/square foot (design) and 1,500 GPD/SF (Peak); Weir Loading @ Peak Hourly=20,000 GPD/linear foot;
Use Design Flow=1,000,000 GPD (1.55 CFS); Peak Hourly=2,500,000 GPD (3.87 CFS);
Design=1,000,000 GPD/1,000 GPD/SF=1,000 SF; Peak=2,500,000/1,500=1,667 SF
Typical design seeks a length about 3 times the width so, 1,667 SF=24′ wide×70′ long×10′ deep; Forward velocity=3.87 CFS/(10′×24′)=0.016 Ft. per Second (FPS).
An EPA study provided a summary of settling data from multiple wastewater plants. The table below is an average of pertinent findings to support the design parameters as they relate to BOD reduction:
OrganicAverageSuspended% Primary(BOD)Settling% >50% BODSolidsSewageContentVelocitymicronsReductionSettleable4550%0.106 FPS64%22.5%  (>100 microns)Supracolloidal3530%68%0%(1-100 microns)Colloidal2020%0%0%(0.2-1.0 microns)
The values in the above table are averages taken from several WWTP that include storm water, combined sewer systems, and sanitary sewage. The settleable solids have a settling velocity range from 0.016 to 0.115 FPS with an average of 0.106 FPS as stated in the table.
The design example above results in a forward velocity of 0.016 FPS which is less than the average settling velocity of 0.106 FPS. The tank is 10′ deep so the solids will settle in 94 seconds. The forward distance travelled in 94 seconds is 1.5 Feet so the solids will settle before the liquid reaches the effluent weir. The EPA study expressed considerable difficult in establishing a consistent average for the supracolloidal and colloidal solids as they vary from site to site and range from 0.0007 to 0.002 FPS. The forward velocity is 0.016 FPS and the tank is 70 Ft long therefore the travel time=4,375 seconds therefore the depth of settling is 3′ to 8.75′.
The effluent weir is 2,500,000 GPD/20,000 GPD/Ft.=a minimum of 125′, the tank is 24′ wide therefore use 3-double sided weirs providing 144′ of weir length so the flow is 2,500,000 GPD/144=17,361 GPD/Ft or 0.027 CFS/Ft. at the weir. The velocity of the liquid at 3′ from the weir is 0.0057 FPS and at 8.75′ the liquid velocity is 0.002 FPS. Some portion of the supracolloidal solids will be removed as per this mathematical exercise on clarifier velocities, but very little of the colloidal solids.
It would be reasonable to expect the primary clarifier in this design example to reduce the BOD to the receiving stream or secondary treatment process by 33%.
Developed and developing nations, as well as the environment, would significantly benefit from removing more than 20-33% of the organic matter from the waste water in the primary treatment because;                Less CO2 would be released to the atmosphere.        Less energy consumed to convert the organic matter (BOD) to biomass (secondary sludge)        Less secondary sludge to pump, store, aerate, dewater, and send to landfill        Fewer trucks hauling secondary sludge to landfill or composting facilities        Landfills would have a longer operational life and release less methane to the atmosphere        Smaller secondary treatment system would be possible resulting in significant capital costs savings for the developed and developing countries allowing more to be done sooner        Lower operational and maintenance costs for the secondary treatment systems        Higher quality primary effluent would accelerate improvements to the receiving waters and reduce environmental health and safety issues        The higher concentration of organics in the primary sludge significantly increases the energy generation potential in anaerobic digesters. Anaerobic Digesters capture and utilize the methane gas created from the high volatile primary sludge to produce energy versus releasing most of the methane to atmosphere due to poor capture systems in landfills.        Waste water treatment plants become a renewable resource recovery facility creating more energy than they consume as the organic load to the secondary treatment process is reduced and the organic fuel for the anaerobic digesters is increased.        Anaerobic Digestion creates less bacteria and results in a Class A sludge that can be used for composting.        
The organic removal rate of primary clarifiers can be improved from 33% to approximately 50% by the addition of coagulating chemicals. This improvement is called Chemically Enhanced Primary Treatment (CEPT) and CEPTs have demonstrated all of the above described benefits. There were no physical or operational modifications to the primary clarifier tank, influent flow baffle, sludge scrapper mechanisms, scum trough or effluent trough. The coagulant forms a floc or gel net that is larger and more dense than the individual suspended solids. As this floc settles it gathers some supracolloidal and colloidal particles thus reducing the BOD and suspended solids flowing to the secondary treatment process.
The Ballasted Floc Reactor (BFR) followed the CEPT in an attempt to remove more BOD and reduce capital costs. The BFR technology removes approximately 50% of the BOD, the same as CEPT, but with a smaller clarifier because the solid settling rate is much higher.
Developing nations would likely not be able to see the benefits of enhanced BOD reduction with the CEPT or BFR products because the chemicals and skilled operators may not be available.
In summary, conventional primary clarifiers, BFRs and CEPTs do not have screened effluent weirs to retain the supracolloidal and colloidal organic particles. Simple placement of a screen at existing effluent weirs will not work because a) such screens would foul in a short time frame due to the high flow velocity at the weir design liquid flow velocities; b) such screens would be stationary so there is no backwashing; and c) such screen would foul due to organic growth on the screen since the screen is in the liquid all of the time. The forward velocity from the inlet to the effluent weir is constant so there is an inertia imparted into the solids keeping them moving towards the effluent weir; there is no velocity control within the tank as the tank is always full so if 10 gallons of liquid enters the tank, 10-gallons of liquid must exit the tank at the same rate as it was added; and the sludge removal equipment in the tank is continually moving and disturbing the settled sludge creating eddies that keep neutrally buoyant constituents and colloidals in suspension moving towards the effluent weir at a high effluent weir entrance velocity.
A screened decanter comprising an effluent weir is disclosed in U.S. Pat. Nos. 7,972,505 and 8,398,864, the relevant disclosures of which are incorporated herein by reference. The movement of a screened decanter is an arc rotating about a pivot. The vertical movement of the screened decanter about a pivot comprises both horizontal and vertical movement in the direction of motion. Depending upon the depth of the tank, the length of the pivot arm requires that the decanter assembly occupy a relatively large footprint in the tank.
What is needed in the art is a screen assembly in the form of a rectangular box or cylinder that is controllably driven in the vertical direction to optimize the exposure of the screen to the wastewater to varying wastewater levels and that can be lifted from the wastewater for backflushing and sterilization in a dedicated overhead apparatus. Because the motion of the screen assembly is only vertical, the required footprint can be relatively small.
What is further needed is an assembly comprising a ganged plurality of such screen box assemblies for wastewater systems having high flows, limited surface area, and/or shallow active tank volumes.
It is a principal object of the invention to provide a high and constant effluent flow rate from a wastewater treatment facility over a wide range of influent flow rates.